1. Field of Invention
The present invention relates to a current source, and more particularly, to a current source circuit with an adjustable temperature coefficient.
2. Description of Related Art
Recently, in analog circuits, along with the progress of processes, the number of transistors contained in a unit area is increasingly larger, such that a large amount of thermal energy is generated during the operation of the circuit, and thus the temperature of circuit will also rise dramatically. Due to the rising temperature, properties of many elements in analog circuit will change, thus the performance of circuit becomes worse. For example, differential pairs frequently appear in analog circuits are connected by sources of two transistors, and the two transistors are driven by a bias current. When the bias current changes due to the variation of temperature, both voltage gain and noise of the differential pair circuit are affected. Therefore, it is desirable to use a reference circuit in analog circuit to generate stable and temperature-free bias current.
Similarly, an operationally stable and temperature-free reference potential is also desired to define the overall range of the input or output potential in analog-to-digital (A/D) converters and digital-to-analog (D/A) converters.
To obtain a stable reference potential not subject to temperature variation, a positive temperature coefficient voltage must be used to compensate a negative temperature coefficient voltage, for example, FIG. 1A illustrates a simplified circuit diagram of a conventional bandgap voltage reference circuit. In FIG. 1A, the base-emitter voltage VBE of ambipolar transistor Q is a negative temperature coefficient voltage. This circuit uses voltage directly proportional to absolute temperature to multiply K and then compensates the negative temperature coefficient VBE, and a zero temperature coefficient voltage Vref is output after addition.
FIG. 1B is an actual layout of the conventional circuit of FIG. 1A, which comprises ambipolar transistors Q1, Q2, Q3, resistors R1, R2, a P-type MOS transistor M3, and current mirrors 10 and 20, wherein the current mirror 10 includes identical P-type MOS transistors M1-M2, and the current mirror 20 includes identical N-type MOS transistors M4-M5. Two identical currents generated by the current mirrors 10 and 20 respectively flow into Q1 and Q2, and the voltages at nodes P1, P2 are identical.
If the base-emitter voltage of ambipolar transistor Q1 is represented as VBE1, and the base-emitter voltage of ambipolar transistor Q2 is represented as VBE2, the voltage drop between the two ends of resistor R1 is VBE1−VBE2, and it is learnt from the physical property of ambipolar transistor that VBE1−VBE2 is a positive temperature coefficient voltage, thus the current flowing through R1 is a positive temperature coefficient current. Moreover, a current mirror structure is formed by using P-type MOS transistors M2, M3, so as to replicate current of resistor R1 to resistor R2, thus the voltage drop between the two ends of resistor R2 is a positive temperature coefficient voltage. Since the base-emitter voltage of the ambipolar transistor Q3 is a negative temperature coefficient voltage and the emitter of the ambipolar transistor Q3 and the resistor R2 are electrically connected, positive and negative temperature coefficient voltages compensate each other, so as to output a zero temperature coefficient voltage Vref.
Conventionally, the output zero temperature coefficient voltage Vref of the bandgap voltage reference circuits tends to be limited to approximate 1.2 volts. If other voltages are preferable, voltage division or other methods must be employed. If a temperature-irrelevant current is desired, and the zero temperature coefficient voltage output by the bandgap voltage reference circuit must be driven by a resistor to generate a zero temperature coefficient current, which makes the circuit become more complicated. The addition of a resistor again results in a further expansion of circuit area and reduces the competitiveness of the integrated circuit.